At the age of 18 he joined the Socialist Party of America and became a devoted follower of Eugene Debs. His friend Tom Kerry claimed that Cannon considered Debs as "one of the greatest orators, agitators, and propagandists that the American working class radical movement had produced."
Cannon was also an organizer for the Industrial Workers of the World (IWW) where he worked under Frank Little, who was lynched in 1917. Cannon also got to know Vincent Saint John. He later recalled: "Despite his modesty of disposition, his freedom from personal ambition, and his lack of the arts of self-aggrandizement, his work spoke loudly and brought him widespread fame."
According to his friend Joseph Leroy Hansen: "Fundamentally, Jim was an angry person. He was angry at injustice, at inequities, at special privileges, at exploitation. He was angry at poverty, lack of opportunity, oppression, racism, and sexism."
The right-wing leadership of the Socialist Party of America opposed the Russian Revolution. However, those members who disagreed with this policy formed the Communist Propaganda League. In February 1919, Jay Lovestone, Bertram Wolfe, Louis Fraina, John Reed and Benjamin Gitlow created a left-wing faction that advocated the policies of the Bolsheviks in Russia. On 24th May 1919 the leadership expelled 20,000 members who supported this faction. The process continued and by the beginning of July two-thirds of the party had been suspended or expelled.
In September 1919, Cannon, Jay Lovestone, Earl Browder, John Reed, James Cannon, Bertram Wolfe, William Bross Lloyd, Benjamin Gitlow, Charles Ruthenberg, William Dunne, Elizabeth Gurley Flynn, Louis Fraina, Ella Reeve Bloor, Rose Pastor Stokes, Claude McKay, Martin Abern, Michael Gold and Robert Minor, decided to form the Communist Party of the United States. Within a few weeks it had 60,000 members whereas the Socialist Party of America had only 40,000.
George Novack later recalled: "He (Cannon) became one of the founders of the American Communist Party, was elected to its Central Committee in 1920, and edited its first national organ. He sharply disagreed with those ex-radicals and academic historians who disparaged the pioneer days of American Communism. He regarded its formative period as a salutary step in the development of the movement toward a socialist revolution in the United States." Cannon argued: "Allowing for all its mistakes and the inadequacies of its leadership, the party that responded to the Russian revolution was the first genuinely revolutionary political party in this country"
Initially, the American Communist Party was divided into two factions. One group that included Charles Ruthenberg, Jay Lovestone, Bertram Wolfe and Benjamin Gitlow, favoured a strategy of class warfare. Another group, led by Cannon, William Z. Foster and William Dunne believed that their efforts should concentrate on building a radicalised American Federation of Labor.
Charles Ruthenberg was appointed as National Secretary of the party. As the author of The Roots of American Communism (1957) pointed out: "Ruthenberg was the natural choice for National Secretary of the Communist party for two reasons - he was a native-born American, and he had demonstrated his ability to run an organization. Almost no one else qualified on both counts." Cannon was elected to the Central Committee and served as District Secretary for the states of Kansas, Missouri and Nebraska. He also became editor of The Worker's World.
The growth of the American Communist Party worried Woodrow Wilson and his administration and America entered what became known as the Red Scare period. On 7th November, 1919, the second anniversary of the revolution, Alexander Mitchell Palmer, Wilson's attorney general, ordered the arrest of over 10,000 suspected communists and anarchists. These people were charged with "advocating force, violence and unlawful means to overthrow the Government". Palmer and his assistant, John Edgar Hoover, found no evidence of a proposed revolution but large number of these suspects were held without trial for a long time. The vast majority were eventually released but Emma Goldman, Alexander Berkman, Mollie Steimer, and 245 other people, were deported to Russia.
It was decided that because William Z. Foster had a strong following in the trade union movement that he should be the party candidate in the 1924 Presidential Election. Benjamin Gitlow, was chosen as his running-mate. Foster did not do well and only won 38,669 votes (0.1 of the total vote). This compared badly with the other left-wing candidate, Robert La Follette, of the Progressive Party, who obtained 4,831,706 votes (16.6%).
In 1927, Cannon joined with others such as John Dos Passos, Alice Hamilton, Paul Kellog, Jane Addams, Heywood Broun, William Patterson, William Z. Foster, , Upton Sinclair, Dorothy Parker, Ruth Hale, Ben Shahn, Edna St. Vincent Millay, Felix Frankfurter, Susan Gaspell, Mary Heaton Vorse, John Howard Lawson, Freda Kirchway, Floyd Dell, Katherine Anne Porter, Michael Gold, Bertrand Russell, John Galsworthy, Arnold Bennett, George Bernard Shaw and H. G. Wells in the campaign against the proposed execution of Nicola Sacco and Bartolomeo Vanzetti.
James Cannon was appointed chairman of the American Communist Party. He attended the Sixth Congress of the Comintern in 1928. While in the Soviet Union he was given a document written by Leon Trotsky on the rule of Joseph Stalin. Convinced by what he read, when he returned to the United States he criticized the Soviet government. "I never deceived myself for a moment about the most probable consequences of my decision to support Trotsky in the summer of 1928. I knew it was going to cost me my head and also my swivel chair, but I thought: What the hell-better men than I have risked their heads and their swivel chairs for truth and justice. Trotsky and his associates were doing it at that very moment in the exile camps and prisons of the Soviet Union. It was no more than right that one man, however limited his qualifications, should remember what he started out in his youth to fight for, and speak out for their cause and try to make the world hear, or at least to let the exiled and imprisoned Russian Oppositionists know that they had found a new friend and supporter."
According to Joseph Leroy Hansen: "The document completely convinced Cannon. He decided to battle for Trotsky's criticisms - not because of any hope of immediate success, but because he saw that Trotsky was right. It was not an easy decision. Cannon realized, perhaps better than anyone outside of the Russian Trotskyists, that it would mean ostracism, the breakup of old friendships, and the end of personal relations with many comrades he had known in common battles for years. However, it was politically necessary to make the turn. For Jim this consideration was paramount. Nothing personal could be permitted to stand in the way of moving ahead in defense of Trotsky's position and against Stalin's bureaucratic gang."
As a result of his actions, Cannon, Max Shachtman and Martin Abern were expelled from the party. Cannon now joined with other Trotskyists to form the Communist League of America. He also published the journal, The Militant. in 1934 the party merged with the American Workers Party, to form the Workers Party of the United States, under the joint leadership of Cannon and Abraham Muste. The party was dissolved in 1936 when it was decided that members should join the successful Socialist Party of America.
Claude McKay once claimed that Cannon "had all the magnetism, the shrewdness, the punch, the bag of tricks of the typical American politician." George Novack added: "Cannon represented the crossbreeding of two desirable traits: a passionate plebeian hatred of the crimes and injustices of capitalist society combined with an indomitable adherence to the ideas and traditions of the original Bolshevik leaders. The fusion of these elements in his makeup accounted for the strength of his convictions, the stamina of his revolutionary will, and the persistence of his fight for the truth."
George Breitman recalls meeting Cannon in 1936: "Jim was of medium height and without excess weight for a man of his age. His body was sturdy but starting to go soft; the shoulders had already taken on a slight stoop, and his hair was turning iron gray. His eyebrows were raised in mild surprise, but otherwise he appeared unruffled."
Norman Thomas, the leader of the Socialist Party of America, decided to expel the Trotskyists in 1937. Shachtman and Cannon were part of a delegation sent to Mexico City to discuss the draft Transitional Program of the Fourth International with Leon Trotsky. As Farrell Dobbs has pointed out: "He (Cannon) was always on the alert, however, to build. Jim was always trying to draw into the revolutionary team every individual who was willing to serve. He also had a quality of watching tendencies and trends inside a movement, and of thinking always in the largest possible terms with respect to the recruitment of cadres. He recognized, as all serious revolutionaries must, the importance of cadres, the value of cadres, and the indispensability of cadres, and what a crime it is when people cavalierly destroy cadres, or ignore cadres, or let them wither-on the vine, or wander down a bypass without really trying to help them find the revolutionary main road."
Max Shachtman became disillusioned with the Soviet Union when it signed the Soviet-Nazi Pact. These feelings were intensified when the Red Army invaded Poland (September, 1939) and Finland (November 1939). Cannon continued to support the foreign policy of Joseph Stalin. Cannon, like Leon Trotsky, believed that the Soviet Union was "degenerated workers' state", whereas Shachtman argued that Stalin was developing an imperialist policy in Eastern Europe. Shachtman decided to leave the Socialist Workers Party and establish his own Workers Party. Other members included Martin Abern, C.L.R. James, Hal Draper, Joseph Carter, Julius Jacobson and Irving Howe.
An opponent of America's involvement in the Second World War, Cannon was arrested under the Alien Registration Act in 1941. He was detained in Sandstone Prison in Minnesota and was not released until 1945. At the 1946 Socialist Workers Party convention he said: "He who doubts the socialist revolution in America does not believe in the survival of human civilization, for there is no other way to save it. And there is no other power that can save it but the all-mighty working class of the United States."
Cannon remained as national secretary of the Socialist Workers Party until replaced by Farrell Dobbs in 1953. Books by Cannon include Struggle for a Proletarian Party (1943), History of American Trotskyism (1944), America's Road to Socialism (1953) and Speeches for Socialism (1971).
James Cannon died on 21st August 1974.